This invention is concerned with improvements in or relating to explosion relief means, particularly for large processing plants in which finely divided materials are subjected to heat treatment.
2. Description of the Prior Art
In many processing plants, particularly large processing plants involving heat treatment of materials which are in a finely-divided granulated or dust-like state, it is a requirement that pressure relief means be provided which is operative in the event that the pressure within the plant rises substantially above the working pressure, to minimise damage caused to the plant in the event of an internal explosion.
In plant operating at low internal pressure (e.g. 10 lbs/sq.ft) the pressure relief means is conveniently in the form of one or more doors which are normally closed, but which open automatically in the event of a rise in pressure within the plant above a predetermined maximum safe pressure.
However, in certain types of processing plants, particularly those operating at high temperature, and at high internal pressure, (viz. upwards of 60 lbs/sq.ft) difficulties are encountered by the use of doors as pressure relief means. One of the difficulties is that it is necessary to vent the excess pressure rapidly over a wide surface area, thus necessitating the provision of large pressure relief openings (for example, upwardly of 2 foot in diameter) and consequently large pressure relief doors. In addition, in a plant which is operating at high internal working pressures (greater than 60 lbs/sq.ft) it is necessary to provide robust securing means to retain the door closed against the normal working pressure of the plant.
For this reason, one is faced with the alternative, either of providing a relatively heavy, and physically stable door which may be sealed completely around the relief opening with a low pressure, or a lighter door which, because of a tendency to "move" upon its mounting, requires a higher pressure to obtain a satisfactory seal against the internal working pressure. However, heavy doors are also disadvantageous, because their momentum causes them to take a long time to be moved to their open positions, and are consequently less effective in venting an internal plant explosion.
It is additionally desirable to provide explosion relief means which comes into operation at as small an excess over working pressure as possible, for example, a relief means which comes into operation at twice the working pressure. This necessarily imposes a limitation on the sealing pressure which may be applied to the doors upon closure thereof.
Heretofore, it has been necessary to a large extent to provide pressure relief doors which are operable at a pressure higher than that which would otherwise be preferred, in order to obtain satisfactory sealing of the doors around the relief openings.
As an alternative to the use of doors, there may be provided as pressure relief means panels which will be ruptured, broken or made otherwise ineffective in containing the atmosphere within the plant when the pressure within the plant rises substantially above the working pressure.
However, it has been found that, in for example coal pre-heating plants, which typically operate at a temperature of 260.degree. C. a pressure of 290 lbs/sq.ft., when an internal explosion occurs, rendering the explosion relief means operative, subsequent to the explosion, and during the period of time whilst the plant is being run down, any air drawn into the plant may cause a further explosion, and that this further explosion may be more violent than the initial explosion.
For these reasons, there has been a tendancy to rely upon safety features incorporated into the operational control of the plant, under which (for example) the plant is shut down automatically should the oxygen content in the atmosphere within the plant rise above a level which is considered to be a safe limit.
This is inconvenient, and may cause the plant to be shut down unnecessarily, or unnecessarily frequently. In addition, because of the length of time taken for the plant to run down, it may be necessary to provide some explosion relief in the event of an explosion occurring within the plant during run down.
It is one of the various objects of this invention to provide explosion relief means for use in a plant operating at high internal pressures, which may conveniently be closed to provide an adequate seal against escape of the internal atmosphere of the plant, but will readily move to an open position to vent any build up of pressure within the plant.